1. Field of the Invention
The present invention relates to a surface acoustic wave motor utilizing ultrasonic vibration, and in particular to an input frequency control device for a surface acoustic wave motor utilizing ultrasonic vibration and provided with an elastic member and a piezoelectric member for vibrating said elastic member.
2. Related Background Art
The U.S. Pat. No. 4,510,411, issued Apr. 9, 1985, discloses an "ultrasonic motor" utilizing ultrasonic vibration and a "surface acoustic wave" motor in which a surface acoustic wave generated on the surface of an elastic member for example with piezoelectric elements drives a rotor maintained in contact with said surface, as actuators replacing the conventional electromagnetic motors. For driving such surface acoustic wave motor with a high efficiency, it has been considered necessary to match the frequency of input power supply with the resonance frequency of a stator consisting of an elastic member and piezoelectric elements.
However the present inventors have found that, in a control to drive a surface acoustic wave motor at such frequency, said resonance frequency itself fluctuates due to a variation in the torque and in various ambient conditions, therefore resulting in difficulties that (1) the motor shows unstable behavior and is unable to provide stable performance, (2) no suitable means is easily available for detecting and maintaining said resonance frequency, and (3) in particular, the performance of the motor is significantly deteriorated when the input frequency becomes lower than the resonance frequency due to the fluctuation thereof.